In the prior art, it is common practice for heavy duty vehicle seats to be constructed on a sub-base with a pivoted parallelogram linkage and a compressed air actuator supporting the seat cushion in position above a fixed base, and with an adjusting valve to regulate the air supply to the air actuator, thereby permitting the occupant of the seat to adjust the ride height of the seat. Such prior art vehicle seats as described above provide a wide range of height adjustment, and the ability to absorb much more of the energy from severe jolts than is possible with a simple rigid seat with foam padding or with air bladder style seat cushions. Operators of trucks and off-highway mobile equipment are required to spend many hours each working day subjected to continual bouncing and vibration, while in a sitting position, resulting in a high incidence of related injuries such as chronic lower back pain and circulatory deficiencies among such workers. In a seated posture, the occupant's body weight is centered over the pelvis, while the legs extend laterally forward of the torso. The pressure between the seat cushions and the driver's body is distributed most predominantly in the area of the pelvis, thus transmitting most of the seating pressure directly through the pelvis directly into the spine. As seat cushions conform to the shape of the occupant's body parts, which are in supportive contact with the seat cushions, there is a reduction of blood circulation in those areas, and fatigue in the joints such as the lower vertebrae, which are subjected to continual compression. Furthermore, because the body weight is supported mainly through the pelvis, the weight of the occupant's legs and lower torso adds to the total weight supported by the base of the spine.
In the prior art, various seats with a plurality of portions which can be varied with respect to form and hardness have been proposed. Such seats are disclosed in U.S. Pat. No. 5,263,765, Hideyuki Nagashima, and U.S. Pat. No. 5,320,409, Kazuhito Katoh, in which a plurality of inflatable bladders are strategically positioned within the seat and are selectively inflated and deflated under the control of a fatigue sensing means. Such seats could offer relief from fatigue caused by continual seat pressure and vibration, but they do not address the absorption of large impacts related to vehicle operation on bumpy terrain and highways.